This invention relates generally to high speed digital telecommunication systems. The invention more particularly relates to alarm indication signals whose generation is required according to proposed standards for the synchronous optical network (SONET).
The telecommunications network servicing the United States and the rest of the world is presently evolving from analog transmission to digital transmission with ever-increasing bandwidth requirements. Fiber optic cable has proved to be a valuable tool of such evolution, replacing copper cable in nearly every application from large trunks to subscriber distribution plants. Fiber optic cable is capable of carrying much more information than copper with lower attenuation.
In attempting to accommodate the protocols, equipment, and cables of the past while providing for the direction of the future, various standards and system requirements relating to fiber optic cables have been adopted. In particular, the T1 Standards Committees of ANSI have provided a draft document ANSI T1.105-1988 dated Mar. 10, 1988 which sets forth specifications for a rate and format of signals which are to be used in optical interfaces. Additional details and requirements are set forth in Technical Advisory publications SR-TSY-000202, --000233, -000253, -000303 Issue 3 of Bell Communication Research (Bellcore). The provided specifications detail the SONET standard. SONET defines a hierarchy of multiplexing levels and standard protocols which allow efficient use of the wide bandwidth of fiber optic cable, while providing a means to merge lower level DS0 and DS1 signals in a common medium. In essence, SONET establishes a uniform, standardized transmission and signaling scheme which provides a synchronous transmission format that is compatible with all current and anticipated signal hierarchies. Because of the nature of fiber optics, expansion of bandwidth is easily accomplished.
A basic SONET signal, termed an STS-1 signal, is seen in FIG. 1. The SONET signal is a 51.84 Mhz, bit serial signal, having nine rows of ninety columns of eight bit bytes at a 125 microsecond frame rate. The first three columns of bytes in the SONET signal are termed the transport overhead (TOH) bytes which include section overhead and line overhead and are used for various control purposes as indicated in FIG. 2. The remaining eighty-seven columns of bytes constitute the STS-1 Synchronous Payload Envelope (SPE) as seen in FIG. 3. In the SPE are included path layer overhead bytes as indicated in FIG. 2.
Returning to FIG. 2, it is seen that the first two bytes A1 and A2 of the transport overhead are framing bytes which contain a specified framing pattern allowing synchronization of the basic SONET STS-1 signal. Three other bytes, H1, H2, and H3 form a pointer giving explicit information as to the location of the start of the SONET SPE (i.e. the "SPE phase"). The pointer bytes are required due to the fact that the position of the SPE is not fixed in time in the STS-1 frame, but is allowed to be displaced in time (i.e. a change in the location of the SPE in the frame over time constituted an SPE "phase" shift). Additional specified bytes of the transport overhead of an STS-1 frame are the C1 byte used for frame identification, the D1 through D12 bytes used for section and line data communications, the E1 and E2 bytes used for orderwire (i.e., voice data transmitted from a network element to another network element), the K1 and K2 bytes used for automatic protection switching (APS) signaling between Line level entities, the B1 and B2 bytes used for section error and line error monitoring (BIP-8), and the F1 byte used as desired by the network provider. The Z1 and Z2 bytes of the STS-1 frame are set aside for non-defined functions. Additional details of the use of these bytes may be found with reference to the above-referenced Bellcore publications.
Among other things specified in the proposed standards are requirements for maintenance of the SONET system, including failure detection and reporting. In particular, as set forth in Bellcore document TA-TSY-00253 (Issue 4, February 1989), which is hereby incorporated by reference herein in its entirety, various failure states such as loss of signal (LOS), loss of frame (LOF), loss of pointer (LOP), must be detected and reported. Details of LOS, LOF, and LOP are found at section 6.3.1.1.1, 6.3.1.1.2, and 6.3.1.1.3 of TA-TSY-00253. In response to the detection of failure, and in accord with section 6.3.1.2. of TA-TSY-00253, an alarm indication signal (AIS) must be sent. The particular AIS signal which is sent is dependent on the type of equipment sending the signal. For example, as defined in section 6.3.1.2.1, a Line AIS is sent by a STE (section terminating equipment) to alert a downstream LTE (line terminating equipment) that a failure has been detected. The Line AIS is supposed to be generated within 125 .mu.sec after detection of a failure state by the STE. The signal generated by the STE to alert the downstream LTE of failure is an OC-N signal that contains valid Section overhead and a scrambled all-ones pattern for the remainder of the signal (i.e., for the line overhead bytes and SPE).
A STS-Path AIS is sent by a line terminating equipment (LTE) to a downstream STE within 125 .mu.sec of failure detection, and comprises an all ones signal in H1, H2, H3, and in the entire STS SPE. Details of the STS- Path AIS are set forth in section 6.1.2.2 of TA-TSY-00253. Other alarm indication signals include a VT-Path AIS and a DSn AIS which are specified in sections 6.3.1.2.3 and 6.3.1.2.4 of TA-TSY-00253. FIGS. 6-3 through 6-13 set forth which type of AIS is generated depending upon the type of equipment which is generating the AIS, and the type of failure which is detected.
While alarm indication signal requirements are set forth in the proposed standards, no method or apparatus is set forth for generating those signals in the short time frame allotted. Clearly, such methods and apparatus are required. An obvious solution to quickly generating the signals is to use a "hardwire" arrangement between the two ends of the terminating equipment. The problem with this solution is the requirement of an additional wire, which in cross-connect applications means an additional cross-connect matrix.